Ommatidium

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Ommatidium

The compound eye of insect

Insect

Insects are the biggest class of arthropods and the only ones with wings. They are the most diverse group of animals on the planet. They are most diverse at the equator and their diversity declines toward the poles....
s is composed of units called ommatidia. An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea

Cornea

The cornea is the transparency front part of the eye that covers the Iris , pupil, and anterior chamber. Together with the cilliary muscles, the cornea reflects light, and as a result helps the eye to dilate, accounting for approximately two-thirds of the eye's total optical power....
. Each ommatidium is innervated by one axon

Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
and thus provides the brain

Brain

The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as cnidarian and echinoderm have a decentralized nervous system without a brain, while sponges lack any nervous system at all....
with one picture element

Pixel

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The compound eye of insect

Insect

Cornea

Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
and thus provides the brain

Brain

Pixel

In digital imaging, a pixel is the smallest item of information in an image. Pixels are normally arranged in a 2-dimensional grid, and are often represented using dots, squares, or rectangles....
. The brain forms an image from these independent picture elements. The number of ommatidia in the eye depends upon the type of insect and ranges from just a handful in the primitive Archaeognatha

Archaeognatha

Archaeognatha is an order of wingless insects, also known as jumping bristletails. They are among the least evolutionarily changed insects, appearing in the Devonian period along with the arachnids....
and Thysanura

Thysanura

The insects of the order Thysanura, usually referred to as silverfish, may also be called bristletails, from their three long caudal filaments....
to several hundred in larger Diptera.

Ommatidia are typically hexagon

Hexagon

In geometry, a hexagon is a polygon with six edges and six Vertex . A regular hexagon has Schl?fli symbol ....
al in cross section, and approximately ten times longer than wide. The diameter is largest at the surface, tapering toward the inner end. At the outer surface there is a cornea, below which is a pseudocone which acts to further focus the light. The cornea and pseudocone form the outer 10% of the length of the ommatidium.

The inner 90% of the ommatidium contains six to seven (depending on the species) long thin R cells. The R cells are the photosensing cells. They tightly pack the ommatidium. The portion of the R cells at the central axis of the ommatidium collectively form a light guide, a transparent tube, called the rhabdom

Rhabdom

Rhabdoms are transparent rods, found in the center of each ommatidium in the compound eye of arthropods. These rods are constructed from the eight photoreceptor cells in the ommatidium....
. In certain diptera (the mosquito

Mosquito

Mosquitoes are common flying insects in the family Culicidae that are found around the world. There are about 3,500 species. They have a pair of scaled wings, a pair of halteres, a slender body, and six long legs....
es and flies

Fly

True flies are insects of the Order Diptera , possessing a single pair of insect wing on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax....
) the rhabdom has separated into seven independent rhabdomeres. This has required the rewiring of the eye such that each ommatidium now has seven axons leading from it. The advantage to this arrangement is that it increases the number of picture elements by a factor of seven, without increasing the number of ommatidia.

Since an image from the compound eye is created from the independent picture elements produced by ommatidia, it is important for the ommatidia to react only to that part of the scene directly in front of it. To prevent light entering at an angle from being detected by the ommatidium it entered or by any of the six neighboring ommatidia, six pigment cells are present. The pigment cells line the outside of each ommatidium. Each pigment cell is situated at the apex of the hexagons and thus lines the outside of three ommatidia. Light entering at an angle passes through the thin cross-section of the photoreceptor cell, with only a tiny chance of exciting it, and is absorbed by the pigment cell, before it can enter a neighboring ommatidium. In many species, in low-light situations, the pigment is withdrawn, so that light entering the eye might be detected by any of several ommatidia. This enhances light detection but lowers resolution.

Ommatidium

See also